DC-to-DC converters are known for converting a first DC voltage to a second, regulated DC voltage. Typically, a first DC voltage is converted to a series of AC pulses that are then rectified and regulated to create a second DC voltage. In many instances the AC pulses are input to the primary winding of a transformer and the secondary pulses, which may be at a higher or lower voltage than the input voltage, are rectified and regulated, the regulation often involving variation of the input pulse width. One very efficient DC-to-DC converter design is a full-bridge, resonant power converter. Such a converter is described in Steigerwald U.S. Pat. No. 4,864,479 issued Jul. 4, 1989. The converter of the Steigerwald patent is capable of operating at high frequencies, e.g. 1 MHz, and achieving high power densities. The Steigerwald converter employs a zero-voltage switching technique to produce the DC output voltage. Furthermore, zero voltage switching allows for a highly efficient conversion of power and reduced switching noise.
One way to obtain multiple output voltages from a DC-to-DC converter, such as the aforementioned full bridge resonant converter, is to provide additional windings on the output transformer. One main DC output voltage can be regulated according to the method taught in the '479 patent. In order to obtain auxiliary regulated output voltages, however, a high degree of coupling among all transformer windings is important. At high frequencies, tight coupling among all transformer windings can be difficult to achieve, resulting in output voltages that do not track closely enough. Thus it is desirable to have a regulation means for auxiliary outputs that is not closely coupled to the regulation means for the main output.
One means of regulating auxiliary DC output of a zero voltage switching power supply is described in Steigerwald, U.S. Pat. No. 5,038,264, issued Jun. 11, 1990. The '264 Steigerwald patent employs linear series-pass regulators to regulate the auxiliary voltages. A series-pass regulator, however, may be less efficient than desired, since power is wasted in the drop in voltage from the input to the output of the regulator. A more efficient scheme is desirable that would employ a switching regulator to regulate each auxiliary output and would advantageously synchronize the auxiliary regulation with the switching waveform of the secondary side of the main power transformer.